Crystalline polypropylene, which is excellent in stiffness and heat resistance, has a drawback that it has a poor impact resistance, particularly one at a low temperature.
As the method for solving the drawback, a process for polymerizing stepwise propylene and ethylene or other olefins to form a block copolymer, that is the chemical blending method, is well known for example in Japanese Japanese Patent Publication Nos. 11230/1968, 16668/1969, 20621/1969, 24593/1974, 30264/1974, Japanese Patent Laid-Open Publication Nos. 25781/1973, 115296/1975, 35789/1978, 110072/1979, and the like. However, when propylene and ethylene are polymerized in two steps or in multi-steps in order to prepare a rubbery polymer which is to be used as a impact resistant ingredient in the resultant copolymer, the impact resistance is improved, but the product has the drawback that a large amount of polymers having a low crystallinity are produced as by-products due to the presence of copolymeric portions in the product
It has generally been practiced to increase the production ratio of rubbery polymers for improving the impact strength of the block copolymers, but the increase in the amount of the rubbery polymer may often cause the increase of by-products, the adhesion of polymer particles to each other or the adhesion of the polymer particles to the internal wall of a polymer production unit, and thus result in the difficulty of stable continuous operation of the unit for a long period.
In the production of a conventional block copolymer comprising a block of a stereoregular polypropylene, a TiC.sub.13 solid component or a solid component comprising magnesium, titanium and a halogen, but the block of propylene/ethylene copolymer obtained with the catalyst generally has a low uniformity of copolymerization and thus normally has a copolymerizability ratio (r.sub.P r.sub.E) of 1.5 or more. Thus, even if the production ratio of the rubbery copolymer is increased during the copolymerization carried out with the very catalyst, the impact strength desired may be improved only insufficiently. Improvements have thus been proposed in that rubbery polymers having a excellent uniformity in copolymerization are separately prepared with an appropriate catalyst selected for the purpose and the rubbery polymers prepared are then blended with polypropylenes, as shown in Japanese Patent Laid-Open Publication Nos. 136735/1976, 222132/1983, 12742/1986 and 150343/1988.
The blending of such rubbery polymers having a good uniformity in polymerization may, however, entail problems that the rubbery copolymer is expensive and an operation is required for the blending.
Furthermore, the conventioal multi-step polymerization is ordinarily a two-step polymerization, and the rubbery polymer, which is described to be produced in either the former or latter step, is actually produced in the latter step in working examples of the proposed methods. It is quite reasonable to produce the rubbery polymer in the latter step, because in polymerization employing a titanium-containing solid catalyst component, the so-called Ziegler polymerization, if the rubbery polymer is produced in the former step, desired impact resistant polymers would not be in fact produced due to the facts that the rubbery polymer will prevent the catalyst from being uniformly distributed or that the rubbery polymer will not be dispersed uniformly in the final block copolymer.
Recently, it has been proposed to prepare a crystalline polypropylene in liquid propylene in the former step and to copolymerize propylene and ethylene or an .alpha.-olefin having 4-20 carbon atoms in the latter step for the purpose of improving the impact strength at a low temperature in EP 33989, EP 433990 and Japanese Patent Laid-Open Publication No. 114050/1992. It seems, however, difficult to produce a stable copolymer by these proposed methods, in that, while impact resistance at a low temperature may be improved, finely divided particulate polymers may tend to be produced, and when the amount of the polymer produced in the latter step is increased the agglomeration of particles or their adhesion to the wall of a reactor may tend to take place.
The object of the present invention is to produce a propylene block copolymer having an improved impact resistance without the problems in the conventional techniques such as the necessity of blending a rubbery copolymer having a good uniformity, the generation of finely divided particles, the agglomeration of particles or their adhesion to the wall of a reactor.